The present invention relates to cellular and satellite communications. More particularly, the invention relates to methods and apparatus for efficient implementation of communication systems using constant envelope offset quadrature shift key (CEOQPSK) and minimum shift key (MSK) modulated carriers in very small aperture terminal (VSAT) satellite applications with improved transmission carrier spectral efficiency.
Recently, VSAT earth stations which employ very small antennas and cost effective transponder circuitry have become increasingly acceptable with the general reduction in the overall cost for the VSAT earth station concept. The VSAT antenna ground terminals developed recently, referred to as remote ground terminals, have been developed for data transmission at low rates. In such systems, the remote ground terminals are utilized for communicating via a satellite from a remote location to a central hub station. The central hub station communicates with multiple remote ground terminals, and has a significantly larger antenna, as well as a significantly larger power output capability than any of the remote ground terminals.
VSAT remote terminals may be used to communicate data, voice, and video, to or from a remote site to a central hub. Typically, the VSAT remote terminals have a small aperture directional antenna for receiving from or transmitting signals to a satellite, via an outdoor unit (ODU) mounted near the antenna for transmitting a modulated carrier generated by an indoor unit (IDU). The IDU demodulates incoming signals received from the ODU and also operates as an interface between a user communication equipment and the ODU of the remote ground terminal.
The viability of the remote ground terminal concept increases as the cost for providing a remote ground terminal at the remote location decreases. In pursuit of this objective, various techniques have been utilized to reduce the cost of the remote ground terminal. For example, U.S. Pat. No. 5,208,829 describes a spread spectrum technique that maximizes the power output from a satellite so as to allow a cost saving reduction in the size of the antenna of the remote ground terminal. U.S. Pat. No. 5,309,479 describes a remote ground terminal comprising a low cost transmitter for producing a frequency shift key (FSK) modulated uplink signal which is utilized to communicate with the central hub station via the satellite. Cost effective IDU and ODU design concepts are described in U.S. Pat. No. 5,774,788 which discloses the remote ground terminal having an ODU with a frequency multiplier which allows for operation with a decreased noise component in the output signal transmitted to the satellite.
Previously VSAT remote terminal systems have used linear quadrature phase shift key (QPSK) modulated carriers, which require a linear mixing type ODU for frequency up conversion. The use of linear up converters to frequency convert the modulated data signal contributes to the generation of unwanted spurious signals due to the fact that linear up converters typically contain local oscillators (LOs) which operate in conjunction with mixers to perform the frequency multiplication. Additionally, such circuitry typically requires significant back off of the output high power amplifier (HPA) in order to maintain linearity to provide good carrier spectral efficiency. These systems are complex and expensive to produce, and such circuitry presents numerous drawbacks. Schemes using frequency shift key (FSK), minimum shift key (MSK), and gaussian minimum shift key (GMSK) modulated carriers are also used which are less complex, and lower in cost without the need for linear up conversion IDUs, however the penalty paid with schemes such as FSK, MSK, and GMSK is considerably reduced carrier spectral efficiency.
Accordingly, in order to reduce the cost and increase the efficiency of VSAT systems, there exists a need for remote ground terminals having an IDU utilizing a modulated carrier which has the advantages of the lower cost schemes while also producing carriers with low noise characteristics while providing good spectral efficiency comparable to linear QPSK. There also exists a need to minimize the number of circuitry components contained in the IDU and ODU system to provide VSAT transponder circuitry in a cost effective manner.